Story Problem: You have seven apples. Someone gives you a few more apples, but you don't know how many they give you. All you know is that once they leave, you realize that you now have 9 apples total. How many apples did this person give you?
To answer this question, you can guess and check to see that the answer is 2. This is because 7+2 = 9. To use algebra, let x be the number of apples that the person gives you. So we have 7+x = 9 which is the same as x+7 = 9. Subtract 7 from both sides (to keep things balanced) and we end up with
x+7 = 9
x+7-7 = 9-7
x+0 = 2
x = 2
So this is a more formal way to find the answer is 2
